Single word to change groups

Solution 1:

Defect! might work:

1 : to forsake one cause, party, or nation for another often because of a change in ideology
// a former KGB agent who defected to America

_{(source: Merriam-Webster)}

and another verb which conveys the same idea is to renegade:

: to become a renegade (a deserter from one faith, cause, or allegiance to another)

_{(source: Merriam-Webster)}

but I'm not sure if this can actually be used in imperative form.

Of course, your Game Master is free to have the enemy interpret either option as taking the sides of another (third) party.

Solution 2:

The trickiness here is that you're asking for a verb to do two distinct functions:

1. Be your ally.

2. Be their enemy.

1 is especially tricky because customarily English would use a grammatical object to signal who should be allied. Omitting the direct object from "Befriend me," "Guard me," and "Protect me" would either lead to ambiguity ("Befriend" and "Protect" could default to a number of targets) or a less productive action ("Guard" - the target would take a defensive stance).

However, if you wanted (2) the target to oppose their masters (so "their enemies would be your enemies"), try revolt. It is intransitive. Merriam-Webster:

: to renounce allegiance or subjection (as to a government) : REBEL

And here's the also-good choice of rebel:

: to renounce and resist by force the authority of one's government

In other words, to revolt or to rebel have a strong sense of using violence ("resist by force") against the people one once owed allegiance to. That sense is strong enough that a "bloodless revolution" is a remarkable event. A character who took that command and didn't attack their former soldiers or leaders would be taking an unusual step.

Solution 3:

Betray Convert Realign Turn Forsake Spurn Revolt

All these basically mean leave one side and join another.